1. Field of the Invention
The present invention relates to a varactor circuit and a tuning method thereof, and more particularly to a linear tuning varactor circuit and a tuning method thereof.
2. Description of the Related Art
Among communication systems, voltage controlled oscillators (VCO) have been widely used to process different signals. In typical communication system, information signals are carried by high-frequency carrier waves for transmission. Through carrier waves with different frequencies, a variety of signals are transmitted. Receivers of communication systems use VCO for extracting signals from the carrier waves. In addition, VCO should be controlled to generate different signals with different frequencies corresponding to the variation of the frequency of the carrier waves.
Because varactors can control the capacitance by controlling the voltage, VCO usually uses LC circuits, composed of varactors and inductances, which can vary the oscillation frequency thereof by tuning the capacitance of the varactors.
FIG. 1A is a prior art single-end varactor circuit. Referring to FIG. 1A, the single-end varactor circuit 10 comprises a varactor 11, a tuning terminal 13, a reference voltage terminal 14 and a connecting terminal 16. The tuning terminal 13 is adapted to receive the tuning voltage Vtune, and the reference voltage terminal 14 is adapted to receive the reference voltage Vdc with a fixed voltage. A terminal of the varactor 11 is coupled to the connecting terminal 16, another terminal is coupled to the reference voltage terminal 14, and the connecting terminal 16 is coupled to the resistor 12 coupled to the tuning terminal 13.
FIG. 1B is a prior art differential varactor circuit. Referring to FIG. 1B, the differential varactor circuit 20 comprises a first single-end varactor circuit 20a and a second single-end varactor circuit 20b. The first single-end varactor circuit 20a comprises a connecting terminal 19, a tuning terminal 28, a corresponding tuning terminal 22 and a varactor 17. The tuning terminal 28 is adapted to receive the tuning voltage Vtune, and the corresponding tuning terminal 22 is adapted to receive the corresponding tuning voltage Vtune,N. The resistors 21, 23 and 25 are adapted to receive DC voltages and exclude AC signals. A terminal of the varactor 17 is coupled to the connecting terminal 19, another terminal is coupled to the corresponding tuning terminal 22, and thetuning terminal 28 is couple to the connecting terminal 19 through the resistor 21. The second single-end varactor circuit 20b comprises a connecting terminal 24, a tuning terminal 27, a corresponding tuning terminal 29 and a varactor 18. The tuning terminal 27 is coupled to the tuning terminal 28 of the first single-end varactor 20a, and the corresponding tuning terminal 29 is coupled to the corresponding tuning terminal 22 of the first single-end varactor 20a. A terminal of the varactor 18 is coupled to the corresponding tuning terminal 29, another terminal is coupled to the connecting terminal 24, and the connecting terminal 24 is coupled to the tuning terminal 27 through the resistor 25.
FIG. 2A is a C-V curve showing the relationship of the tuning voltage and the capacitance of an idea varactor circuit. Referring to FIGS. 1A and 2A, the capacitance of the varactor 11 of the varactor circuit 10 is determined by the reference voltage Vdc and tuning voltage Vtune. The tuning voltage Vtune is an adjustable voltage, and the DC voltage Vdc is fixed. The tuning range V1 of the tuning voltage is:V1=Vtune,M−Vdc
Wherein Vtune,M is the maximum tuning voltage.
Theoretically, the curve 31 of the varactor circuit is linear. It means that each of the tuning voltages Vtune corresponds to different capacitances Cvar of the varactor circuit.
FIG. 2B is a C-V curve showing the relationship of the tuning voltage and the capacitance of a prior art varactor circuit. The curve 33 of the varactor circuit is not completely linear, but has a linear region A therein. If the tuning range V2 is within the linear region A, the varactor circuit will operate perfectly. If, however, the tuning range V2 is within the saturation region B, the capacitance Cvar barely changes no matter how to adjust the tuning voltage Vtune.
For the design of VCO, a wide linear controlling region is desired to generate a small C-V gain,       (                  Δ        ⁢                                  ⁢                  C          var                            Δ        ⁢                                  ⁢        V              )    ,under the same tuning range of the capacitance ΔCvar. It means that a small voltage oscillation gain,       (                  K        VCO            ≡                        Δ          ⁢                                          ⁢          f                          Δ          ⁢                                          ⁢          V                      )    ,is desired. With the comparison of FIGS. 2A and 2B, FIG. 2A generates a C-V gain,       (                  Δ        ⁢                                  ⁢                  C          var                            Δ        ⁢                                  ⁢        V              )    ,smaller than that of FIG. 2B because of the wider linear tuning range. Therefore, it is the circuit that generates a small voltage oscillation gain,       (                  K        VCO            ≡                        Δ          ⁢                                          ⁢          f                          Δ          ⁢                                          ⁢          V                      )    .
U.S. Pat. No. 6,563, 392 granted to Ramon Alejandro Gomez disclosed a parallel method for generating the linear range. However, additional accurate power supplies are required for generating the circuit with the desired performance.